Authorities have named the 14 victims of the attack on a social services centre in San Bernardino, California. Twenty-one more people were injured.
Mostly county employees, a group of people gathered on Wednesday for a holiday party with no indication of what was to come from their co-worker Syed Rizwan Farook.
Survivors expressed shock that Farook, a quiet employee of the county for years, could orchestrate such an attack.
Bomb equipment, weapons and thousands of rounds of ammunition were found by police in a raid after a shootout that killed the two suspects.
Authorities still have not found a motive in the attack by Farook, 28, and his wife or fiance Tashfeen Malik, 27.
However, US officials have told the media Tashfeen Malik pledged allegiance to the leader of the Islamic State (IS) group on Facebook.
Here are the victims and their stories.
Yvette Velasco, 27
Yvette Velasco worked as an environmental health specialist for the San Bernardino County Department of Public Health.
"We are devastated about what happened, and are still processing this nightmare," her family said in a statement.
Her uncle told KPCC station: "Yvette was an intelligent, motivated and beautiful young woman, who was full of life and loved by all who knew her."
Larry Kaufman, 42
Larry Daniel Kaufman worked at an independent coffee shop at the Inland Regional Center.
His boyfriend, Ryan Reyes, repeatedly called him after hearing about the shooting. Reyes received conflicting reports about Kaufman all day, ultimately learning he was killed, the Los Angeles Times reports.
He had been texting with him all morning and had dropped him off at work that day. The couple had been together for three years.
Eileen Richey, the executive director of the Association of Regional Center Agencies, said Kaufman's death "fills us with sorrow, knowing so many families are in mourning."
Damian Meins, 58
Meins worked for the county for 28 years.
"Damian was a good, good guy. Funny and smart and tall. A real quick smile, very friendly and outgoing," his friend Walter Hackett said. "It's a real hard hit for all of us who knew him."
"My heart is heavy this morning as we learned that one of our former teachers, Damian Meins, was one of the victims that lost their lives yesterday in the senseless mass shooting in San Bernardino," Laurie Lee Nusbaum wrote on Facebook. "Mr Meins was such a kind man who always had a smile for everyone and dressed as Santa for our school Christmas pictures."
He was married to his high school sweetheart and had two daughters.
He graduated from the University of California at Riverside in 1979.
Sierra Clayborn, 27
Sierra Clayborn was employed as an environmental health specialist for San Bernardino County.
She was energetic, thoughtful and always smiling, Mary Hale, manager of the Fontana apartment complex where Clayborn had lived, was quoted as saying by the Los Angeles Times newspaper.
She went to the University of California at Riverside, graduating in 2010.
The manager of a pizza shop in Big Bear Lake, California said she had just visited to inspect the restaurant.
"She was one of the nicest health inspectors that we've ever had," Matthew Peairs said. "She talked to us like normal people, not just doing her job."
Nicholas Thalasinos, 52
Thalasinos, who had been working as a health inspector, had a growth removed from his head just four days before being killed in the shooting, the Los Angeles Times reports.
"He had an incredibly good work ethic," Ed Beck, the husband of one of his former colleagues, said. "The job of a sanitary inspector is certainly not the most glamorous of professions. He was passionate about it. He wanted to make sure people were safe."
He has two adult sons.
"I want answers and I want them now, because now, it's personal!" wrote friend Yael Zarfi-Markovich on Facebook.
He was a Messianic Jew who often defended Israel. According to the AP, he had got into a heated discussion with Farook about whether Islam was a peaceful religion while working a few weeks ago.
His friend Kuuleme Stephens said she did not know whether those debates were a factor in the attack.
Mike Raymond Wetzel, 37
Wetzel, a father of young children, was working as an environmental health specialist.
His church confirmed on its website that he was one of the victims.
"Michael was the most amazing person," his wife Renee Wetzel said in a statement.
"He was my best friend and an incredible father who was loved by all. I have never known a better person. He loved his work and his family so very much. Without him, this family will never be the same. We appreciate all the love and support that everyone is showing."
Robert Adams, 40
The married father of a 20-month-old daughter worked as an environmental health specialist with San Bernardino County.
His responsibilities included inspections of pools and food facilities during construction in the area.
"Throughout the day and night of 2 December, we prayed, hoped and pleaded with God to somehow spare the life of the remarkable Robert Adams; however, during the senseless mass shooting in San Bernardino, we lost Robert," his friends said.
He and his wife had been planning a trip to Disneyland for their daughter, Savannah.
"Our worst fears were confirmed today: our beloved Robert will not be coming home to us. He was a loving son, brother, husband, and daddy to Savannah," his family said in a statement.
Bennetta BetBadal, 46
Born in Iran, BetBadal fled to America when she was 18 to escape Islamic extremism and the persecution of Christians that followed the Iranian revolution, her relatives said.
She initially settled in New York, but later married a police officer and moved to Rialto, California.
A university graduate in chemistry, she worked as an inspector for the San Bernardino County Health Department.
She leaves three children.
"It is the ultimate irony that her life would be stolen from her that day by what appears to be the same type of extremism that she fled so many years ago," her family said in a statement.
Isaac Amanios, 60
Amanios is survived by his wife and three adult children, according to a family member who asked not to be named, the Los Angeles Times reports.
The family member described him as "an amazing father, brother and amazing everything".
National Football League Player Nat Berhe wrote on Twitter that Amanios was a relative.
"Just got word that one of my cousins was among the 14 killed yesterday. I'm so sick right now," he wrote.
A witness told USA Today that he was sitting next to Farook before he left.
Aurora Godoy, 26
Godoy was married and the mother of an infant son, according to her aunt Rebecca Godoy, writing on Facebook.
"Yesterday in the shootings in San Bernardino many families were affected. Ours was one of them," she wrote. "We will keep her flame alive so that her young son does not forget his special mother."
Harry Bowman, 46
Harry Bowman was a Pennsylvania native, a father of two and had lived in California for the past 15 years, according to a Pennsylvania TV station WGAL-8.
"We are saddened to hear reports that Pennsylvania-native Harry Bowman was among the victims of the senseless violence in San Bernardino, California," Governor of Pennsylvania Tom Wolfe said in a statement. "On behalf of all Pennsylvanians, Frances and I extend our deepest condolences to the Bowman family and all of our fellow York County residents whom knew him."
Tin Nguyen, 31
According to a GoFundMe page set up for her family, Nguyen was "an incredible person with a contagious smile ... you couldn't help but fall in love with her addicting personality and goofy sense of humour."
Two weeks ago, she attended her cousin's bridal shower, which she had planned.
She came to the US from Vietnam at age 8 and graduated from Cal State Fullerton, and worked as a county health inspector for four years, according to the Los Angeles Times.
The Times reports that she and her boyfriend planned to get married in 2017.
She had planned to have her wedding at a nearby church.
"She promised that no matter what, she would return to have her wedding there, and now we're having a funeral," her mother told the newspaper. "What will become of our lives?"
Juan Espinoza, 58
A former co-worker of Espinoza, Scott Wyatt said that he was "a simple guy, quiet, a person who didn't like attention, just liked to do what he had to do. This hits close to home."
Espinoza was a health inspector with the county. He was born in Mexico and moved to the US more than 20 years ago.
Shannon Johnson, 45
Johnson lived in Los Angeles and commuted to San Bernardino County every day to work as a health inspector.
He lived in Los Angeles with his girlfriend who had worked for the mayor of Los Angeles, Eric Garcetti.
Mr Garcetti's office said in a statement "we have learned that one of our own ... is among those who lost their lives."
He shielded colleague Denise Peraza behind a chair during the shooting, the Atlanta Journal-Constitution reports.
Ms Peraza, who was shot in the back but survived. said Johnson held her and said "I got you".
"I believe I am still here today because of this amazing man," Ms Peraza said.